Technical Field
The present disclosure relates to a high-sensitivity, z-axis micro-electro-mechanical detection structure; in particular for a z-axis MEMS (micro-electro-mechanical system) structure.
Description of the Related Art
Z-axis inertial accelerometers of an MEMS type are known in the art and include micro-electro-mechanical structures sensitive to accelerations acting in a direction orthogonal to a main plane and to the top surface of a corresponding substrate (in addition, it is possible to detect further accelerations acting in the same plane).
FIGS. 1a and 1b show, for example, an MEMS structure of a known type, designated as a whole by reference number 1, belonging to a z-axis inertial structure (which further comprises an electronic read interface, not illustrated, electrically coupled to the above MEMS structure).
The MEMS structure 1 comprises a substrate 2 (for example, of semiconductor material, in particular silicon), having a top surface 2a, and a detection mass 3, of conductive material, for example polycrystalline or monocrystalline silicon, and arranged on top of the substrate 2, suspended at a certain distance from the top surface 2a thereof. The top surface 2a of the substrate 2 defines a plane xy in a cartesian reference system xyz having a first axis x, a second axis y, and a third axis z. The detection mass 3 has a main extension in a plane that, in the rest condition and in the absence of external accelerations or stresses, is acting on the MEMS structure 1, and is substantially parallel to the top surface 2a of the substrate 2, and has a substantially negligible dimension along the third axis z.
The detection mass 3 has a through opening 4 throughout its thickness that has, in top plan view, a substantially rectangular shape elongated parallel to the first axis x, and is arranged at a certain distance from the centroid (or center of gravity) of the detection mass 3. The through opening 4 consequently divides the detection mass 3 into a first portion 3a and a second portion 3b, arranged on opposite sides with respect to the through opening along the second axis y.
The first portion 3a has a larger dimension along the second axis y than the second portion 3b. 
The MEMS structure 1 further comprises a first fixed electrode 5a and a second fixed electrode 5b, of conductive material, for example a metal such as aluminum, arranged on the top surface 2a of the substrate 2, on opposite sides with respect to the through opening 4 along the second axis y. In this way, the first and second fixed electrodes 5a, 5b are positioned, respectively, underneath the first and second portions 3a, 3b of the detection mass 3. The fixed electrodes 5a, 5b have, in a plane parallel to the plane xy, a substantially rectangular shape, elongated along the first direction x and thus define, together with the detection mass 3, a first a second sensing capacitors with plane and parallel faces, designated as a whole by C1, C2 in FIG. 1b, each having a respective rest capacitance.
The detection mass 3 is anchored to the substrate 2 through a central anchorage element 6, formed by a pillar element extending into the through opening 4 from the top surface 2a of the substrate 2, centrally with respect thereto. The central anchorage element 6 is consequently arranged at the same distance from each of the fixed electrodes 5a, 5b along the second axis y.
In particular, the detection mass 3 is mechanically connected to the central anchorage element 6 through two elastic anchorage elements 8, which extend into the through opening 4, with a substantially rectilinear extension, aligned with each other along a rotation axis A parallel to the first axis x. The elastic connection elements 8 are arranged on opposite sides of the central anchorage element 6, between the latter and the detection mass 3, and are configured to be compliant to torsion about their extension direction, thus enabling rotation of the detection mass 3 out of the plane xy, about the rotation axis A.
In use, in response to an acceleration acting in the orthogonal direction z, the detection mass 3, due to its eccentricity with respect to the rotation axis A, turns about the latter, by an inertial effect, in such a way that the first or second portion 3a, 3b approaches the respective fixed electrode 5a, 5b and the other portion 3b, 3a recedes from the other fixed electrode 5b, 5a, generating opposite capacitive variations of the detection capacitors C1, C2. An appropriate interface electronics (not illustrated in FIGS. 1a and 1b) of the structure, electrically coupled to the MEMS structure 1, receives the capacitive variations of the detection capacitors C1, C2, and processes them in a differential way for determining the value of the acceleration acting along the orthogonal axis z.
The MEMS structure 1 of FIGS. 1a and 1b, although advantageously enabling detection of accelerations acting along the orthogonal axis z, enables a limited scalability of the dimensions in the case a high sensitivity is specified, defined as variation in the rotation angle as a function of the variation in the external acceleration.
In fact, the reduction in dimensions of the known structure entails a reduction in the length of the arm (distance between the center of mass of the entire detection mass 3 and the rotation axis A) and thus a reduction in the moment of inertia.